User interface improvements in main map view
- layer menu entries are now toggles, so a layer can be switched off by
clicking on it again
- only one year is selected at a time
Code formatting, slight refactoring
Corrected bug in updateDisplay, where the argument k was compared
with the months object, not with the string "month" as it must
Merge pull request #3 from Sentido-Labs/day-3

User interface improvements in main map view